Electrode material, method for the production thereof, and use of same

ABSTRACT

A material for an electrode, the material for as well as a method of making the material for an electrode comprising or consisting of a compound of formula (1) 
       M2Ni1− x Co x O4+δ
 
       and/or of formula (2) 
       La1− y M y Ni1− x Co x O4+δ
 
     where M represents Pr and/or Nd, 0.0≤x≤0.2, 0.25≤δ≤0.3 and 0&lt;y≤10 0.5.

The invention relates to an electrode material, a method for theproduction thereof and the use of same for fuel cells and forelectrolysis, in particular for high-temperature electrolysis, as an airor oxygen electrode.

Electrolysis is a process in which electric current forces a redoxreaction. It is used for example to extract metals or to producesubstances that would be more expensive or very difficult to obtainusing purely chemical processes. Examples of important types ofelectrolysis are the production of hydrogen, aluminum, chlorine andcaustic soda.

Electrolysis requires a DC voltage source which supplies the electricalenergy and drives the chemical reactions. Part of the electrical energyis converted into chemical energy. Batteries, accumulators or fuel cellsserve exactly the opposite purpose, the conversion of chemical energyinto electrical energy, i.e. they serve as a voltage source.Electrolysis can therefore be used to store energy, for example in theelectrolysis of water, which yields hydrogen and oxygen, which have beenproposed as energy carriers. By reversing the water electrolysis in afuel cell, approximately 40% of the electrical energy originally usedcan be recovered.

For the electrolysis of water, so-called high-temperature (steam)electrolysis (at 700 to 1000° C.) on solid electrolytes is also used.Yttrium-stabilized zirconium dioxide (YSZ) is usually used as the solidelectrolyte. Alternatively, Sc or Ca-doped ZrO Gd or Sm-doped CeO oreven electrolytes with a perovskite structure (e.g. based on LaGaO dopedwith Sr and/or Mg) can be used. Due to the increased operatingtemperature, the required voltage at the thermo-neutral operating pointcan be reduced to 1.30 V, and the current density is 0.4 A/cm².Furthermore, the efficiency is improved.

WO 2008/061782 A2 describes so-called air electrodes in general. Thisprior art relates to a thin and essentially unsupported solid oxide cellcomprising at least a porous layer, an electrolyte layer and a porouscathode layer, with the anode layer and the cathode layer having anelectrolyte material, at least one metal and a catalyst material, andthe total thickness of the thin reversible cell being approximately 150μm or less.

Perovskites for air electrodes are also known, for example from U.S.Pat. No. 7,803,348 B1. This document describes that the oxygen isreduced in the presence of a catalyst at the cathode of an alkalielectrolyte fuel cell. Catalysts of formulaSr_(3−x)A_(1+x)Co_(4−y)B_(y)O_(10.5−z), where the values for x, y and zare within defined ranges and A represents Eu, Gd, Tb, Dy, Ho or Y and Brepresents Fe, Ga, Cu, Ni, Mn and Cr, exhibit high catalytic activityand high chemical stability when used as an oxygen reduction catalyst inalkaline fuel cells.

Nickelates are also known from the prior art. For instance, WO2017/214152 A1 describes a solid oxide fuel cell having an anode, anelectrolyte, a cathode barrier layer, a nickelate composite cathodewhich is separated from the electrolyte by the cathode barrier layer,and with a cathode current collector layer being provided. The nickelatecomposite cathode contains a nickelate compound and a second oxidematerial, which can be an ionic conductor. The composite can furthercomprise a third oxide material. The composite material can have thefollowing general formula(Ln_(u)M1_(v)M2_(S))_(n+1)(Ni_(1−t)N_(t))_(n)O_(3n+1)−A_(1−x)B_(x)O_(y)C_(w)D_(z)Ce_((1−w−z))O_(2−δ), where A and B can be rare earth metalsother than cerium.

Finally, lanthanum cobalt nickelates for use in solid oxide fuel cellsare known from US 2012/0064433. This prior art describes a material fora solid oxide fuel cell, which material comprises a lanthanum metaloxide having a perovskite-like crystal structure and a cerium oxidemetal.

Proceeding from the prior art, the problem addressed by the invention istherefore that of specifying a material for an electrode, in particularfor an air electrode for high-temperature electrolysis, and a method forthe production thereof, in which the electrode is improved in terms ofperformance and service life compared to the electrodes known from theprior art.

The problem is solved according to the invention by a material accordingto claim 1, a method according to claim 9 and the uses according toclaim 14. Advantageous developments of the invention can be found in thedependent claims.

The invention relates to a material for an electrode, the materialcomprising or consisting of a compound of formula (1)

M₂Ni_(1−x)Co_(x)O_(4+δ)  (1)

and/or of formula (2)

La_(1−y)M_(y)Ni_(1−x)Co_(x)O_(4+δ).  (2)

where M represents Pr and/or Nd and 0.0≤x≤0.2, 0.25≤δ≤0.3 and 0<y≤0.5,in particular 0.5.

When such materials are used in electrodes, for example in airelectrodes for high-temperature electrolysis, it has surprisingly beenfound that the electrode is improved in terms of performance and servicelife. The term “air electrode” is common in this field, and inparticular electrodes at which the reaction of oxygen takes place duringthe high-temperature electrolysis are referred to as air electrodes. Theprinciples of high-temperature electrolysis are known to a personskilled in the art. They have been described above. The materialsaccording to the invention can be used in these high-temperatureelectrolysis processes and devices which are known per se, as a resultof which the above advantages are achieved.

In one embodiment, x has the values 0.0, 0.1, or 0.2. In anotherembodiment, δ has the values 0.25, 0.28 or 0.3. If materials accordingto the invention are provided with these values for x and δ, thenparticularly performance-improved electrodes with a particularly longservice life are obtained.

In one embodiment, the material is selected from Pr₂NiO_(4+δ),Pr₂Ni_(0.9)Co_(0.1)O_(4+δ), Pr₂Ni_(0.8)Co_(0.2)O_(4+δ), Nd₂NiO_(4+δ),Nd₂Ni_(0.9)Co_(0.1)O_(4+δ) Nd₂Ni_(0.8)Co_(0.2)O_(4+δ) andLa_(1.5)Pr_(0.5)Ni_(1−x)Co_(x)O_(4+δ), where x and δ are as definedabove. These materials are particularly suitable as electrodes forhigh-temperature electrolysis because they have a particularly goodperformance and service life.

In one embodiment, the material has a perovskite structure, inparticular a layered perovskite structure. This structure has proven tobe particularly favorable in terms of performance improvement andservice life. Such materials having a perovskite structure can beobtained by the method according to the present invention, which will bedescribed below.

In one embodiment, the material for an electrode has an average particlesize of 0.5 μm to 1 μm, for example 0.8 μm to 0.9 μm, or 0.5 μm to 0.6μm. The average particle size can be determined using particle sizedistribution and scanning electron microscopy (SEM). Particularlyfavorable electrode materials are provided using these average particlesizes.

The material according to the invention can be used in any form forelectrodes. For example, it can be in the form of a layer. Inparticular, when the material according to the invention for anelectrode is in the form of a layer, it can further comprise a compoundhaving formula (3)

LaNi_(0.6)Fe_(0.4)O_(3−δ)  (3)

where 0<δ≤0.05. This compound of formula (3) can be applied as a layeron the material according to the invention. The current collection isimproved with the compound of formula (3).

The present invention also relates to a method for producing a materialfor an electrode, in particular as described above, comprising the stepsof

-   -   (a) mixing the oxides of the elements Pr, Nd, Ni, Co, La        according to the desired compound of formula (1) or (2),    -   (b) drying the mixture from step (a), and    -   (c) annealing the mixture at a temperature of 1000° C. to        1400° C. for 4 hours to 20 hours in air.

Examples of the oxides used in step (a) can be Pr₆O_(ii) or Nd₂O₃,La₂O₃, NiO and Co₃O₄. These can be dried in order to largely removewater contained therein. This drying can take place for example at atemperature of approximately 900° C. for e.g. 8 hours to 24 hours, inparticular 12 hours to 16 hours. Each oxide used can be driedseparately. The oxides can then be mixed in appropriate stoichiometricratios corresponding to the desired compound of formulas (1) and (2).This can be done in a manner known per se, in the way that solids areusually mixed, e.g. with a ball mill, in particular with zirconia balls.The speed of the ball mill can be 100 to 250 revolutions per minute, forexample approximately 250 revolutions per minute. This can take placefor 2 hours to 6 hours, in particular approximately 4 hours.Furthermore, the mixing can be carried out in the presence of a liquid.The liquid can act as a liquid phase in which the starting materials aresuspended, e.g. organic liquid phases such as isopropanol, ethanoland/or toluene.

The drying step (b) is carried out to remove the contained liquid. Inone embodiment, the drying in step (b) can be carried out at 18° C. to100° C., for example 70° C. to 90° C., e.g. 80° C., for 8 hours to 24hours, for example 10 hours to 14 hours, e.g. 12 hours.

In one embodiment, the annealing in step (c) can be carried out at atemperature of 1100° C. to 1300° C., for example approximately 1300° C.,for approximately 6 hours to 16 hours, for example approximately 12hours.

In one embodiment, after step (c), the average particle size can beadjusted to 0.5 μm to 1 μm, for example 0.8 μm to 0.9 μm, or 0.5 μm to0.6 μm, e.g. approximately 1 μm. The average particle size can bedetermined using particle size distribution and scanning electronmicroscopy (SEM). These particle sizes can be obtained by means ofconventional comminution methods, for example grinding methods, inparticular with a ball mill. The ball mill can have zirconia balls. Thegrinding can take place in the presence of a liquid. The liquid can actas a liquid phase in which the materials are suspended, e.g. organicliquid phases such as isopropanol, ethanol and/or toluene. The grindingcan take place for 4 hours to 12 hours, for example 6 hours to 10 hours,e.g. 8 hours. The grinding temperature can be from room temperature to60° C.

The resulting powders can be processed with liquids and binders to makepastes. These can be applied to half-cells by application techniquessuch as screen printing, film casting and spraying techniques. Halfcells are known per se and are commercially available. This involves anelectrode to which an electrolyte layer is applied. The paste of thematerial according to the invention for an electrode can then be appliedto the side of the electrolyte layer that faces away from the electrodewhich is already present.

After that, another sintering step can be carried out at e.g. 1100° C.to 1200° C., for example 1150° C., for e.g. 0.5 hours to 2 hours, forexample 1 hour, in air.

In one embodiment, a layer of LeNi₀₋₆Fe_(0.4)O_(3−δ), where 0≤δ≤0.05,can be applied in particular to the layer of the material according tothe invention. This makes it possible to improve the current collection.

The invention also relates to the use of the material according to theinvention, as described above, as an electrode material for fuel cellsand for electrolysis, in particular for high-temperature electrolysis,as an air or oxygen electrode.

The invention will be explained in more detail below on the basis of thedescription without restricting the general concept of the invention. Inthe drawings:

FIG. 1 shows a voltage/voltage density curves for the single cells withPNO, PNCO10 and PNCO20 electrodes at 800 and 900° C.

FIG. 2 shows the resistance of the single cells PNO, PNCO10 and PNCO20at 800° C. and a current density of −1 A·cm⁻² with a 50% H₂+50% H₂O gasmixture.

EXAMPLE: PRODUCTION AND ELECTROCHEMICAL PROPERTIES OF ELECTRODEMATERIALS Preparation of Materials and Characterizations:

Three compositions of each series, namely Pr₂Ni_(1−x)Co_(x)O_(4+δ)(PNCO), Nd₂Ni_(1−x)Co_(x)O_(4+δ) (NNCO) andLa_(1.5)Pr_(0.5)Ni_(1−x)Co_(x)O_(4+δ) (LPNCO), (x=0.0, 0.1 and 0.2) wereproduced according to a solid-state synthesis method. Higher cobaltcontents were not taken into account due to the instability of the layerstructure. The corresponding precursors were Pr₆O₁₁ (Aldrich chem,99.9%), La₂O₃ (Aldrich chem, 99.9%), Nd₂O₃ (Alfa Aesar, 99%), NiO (AlfaAesar, 99%) and Co₃O₄ (Alfa Aesar, 99%). In a first step, the powdersPr₆O₁₁, Nd₂O₃ and La₂O₃ were pre-fired overnight at T=900° C. in orderto remove the water content due to their high hygroscopic character. Theprecursors were weighed according to the composition of the nickelatesand then ball-ground with zirconia balls and isopropanol (VWR, 99.8%)for 4 hours at 250 rpm. After drying overnight at 80° C., annealing wascarried out at 1300° C. for 12 hours in air to obtain a pure phase. At alower sintering temperature, some impurities were detected by the XRD.The sintering conditions of 1300° C./12 h result in well crystallizedpure phases. The powders obtained were comminuted and ground again withzirconium dioxide balls and isopropanol for 8 h with the aim ofobtaining an average particle size of approx. 1 μm (checked by means ofparticle size distribution and SEM).

The δ value at room temperature in air was determined by iodometrictitration and TGA experiments. The powders were first balanced in air upto 1000° C., then cooled to room temperature at a slow rate (2° C.min⁻¹), with this cycle being repeated twice to ensure a stable state ofthe material, i.e. a reproducible oxygen content. A second cycle wasthen carried out under Ar-5% H₂ flow with a very slow heating rate of(0.5° C.·min⁻¹), with decomposition of the material leading to thedetermination of the oxygen stoichiometry after cycling the sample toroom temperature (La₂O₃, Nd₂O₃, Pr₂O₃, Pr₂O₃, metallic Ni and Codepending on the composition). For all series, an increase in the 6value was observed upon cobalt substitution. For example, the 6 valuesobtained are 0.25, 0.28 and 0.30 for Pr²NiO^(4+δ) (PNO),Pr₂Ni_(0.9)Co_(0.1)O_(4+δ) (PNCO10) and Pr₂Ni_(0.8)Co_(0.2)O_(4+δ)(PNCO20), respectively.

Electrochemical Performance and Durability as an Oxygen Electrode:

Electrochemical characterization was carried out with NiO—YSZ-supportedcells (NiO YSZ///YSZ//GDC///electrode, CeramTec®, ASC-10C type). Theoxygen electrode, i.e. the anode layers (nickelates) were depositedusing the screen printing method and sintered in air at 1150° C. for 1h. The sintering temperature (1150° C.) was optimized for the PNCOseries to obtain a controlled homogeneous porous electrodemicrostructure. For the measurement, gold and nickel grids (1,024 cm⁻²mesh) were used as current collectors for the oxygen and fuelelectrodes. The i-V characteristic curve was measured in theelectrolysis mode from OCV to 1.5 V with a 50% H₂O and 50% H₂ gasmixture in the temperature range of 700-900° C. The impedance plots wererecorded at OCV and from 1.0 to 1.5 V with an increase of 0.1 V,potentiostatically controlled with 50 mV AC amplitude, from 106 Hz to10-1 Hz, with an IVIUM VERTEX potentiostat/galvanostat with anintegrated frequency response analysis module.

An increase in cell performance was observed with cobalt substitution.The cell current densities obtained under the applied voltage of 1.5 Vat 900° C. are 2.11, 2.41 and 3.0 A·cm⁻² for PNO, PNCO10 and PNCO20single cells and at 800° C. the current densities are 1.6, 1.8 and 1.9A·cm⁻² for PNO, PNCO10 and PNCO20 single cells (FIG. 1).

The durability experiments were carried out with the nickelateelectrodes containing single cells under SOEC conditions at 800° C. withhigh current density, i.e. −1.0 A·cm⁻², for up to 250 h with 50% H₂O and50% H₂ (FIG. 2). All three cells behave differently during thedurability test and initially show a rapid increase. The PNO cell showsa continuous increase up to 250 h, but the rate of increase is lowerafter 70-80 h. The electrolysis voltage increased from 1.38 to 1.43 Vfor PNO after 250 h, showing the highest total degradation among all.The degradation rate was estimated (in mV·kh⁻¹) by carrying out a linearadjustment in the voltage vs. A time curve was implemented and ˜88mV·kh⁻¹ was found for the PNO cell. However, the other two cells showlower degradation rates, i.e. ˜40 mV·kh⁻¹ and ˜22 mV·kh⁻¹ for PNCO10 andPNCO20 single cells, respectively. It is noteworthy that the PNCO20single cell shows the least degradation after 250 hours underelectrolysis conditions.

Of course, the invention is not limited to the embodiments shown in thefigures. The above description is therefore to be considered asillustrative rather than limiting. The following claims are to beunderstood such that a mentioned feature is present in at least oneembodiment of the invention. This does not preclude the presence ofother features.

1. Material for an electrode, the material comprising of a compound offormula (1)M2Ni1−xCoxO4+δ  (1)and/or of formula (2)La1−yMyNi1−xCoxO4+δ  (2) where M represents Pr and/or Nd, 0.0≤x≤0.2,0.25≤δ≤0.3 and 0<y≤0.5.
 2. Material according to claim 1, wherein x hasthe values 0.0, 0.1 or 0.2.
 3. Material according to claim 1, wherein δhas the values 0.25, 0.28 or 0.3.
 4. Material according to claim 1,wherein it is selected from Pr2NiO4+δ, Pr2Ni0.9Co0.1O4+δ,Pr2Ni0.8Co0.2O4+δ, Nd2NiO4+δ, Nd2Ni0.9Co0.1O4+δ Nd2Ni0.8Co0.2O4+δ andLa1.5Pr0.5Ni1−xCoxO4+δ.
 5. Material according to claim 1, wherein thematerial has a perovskite structure.
 6. Material according to claim 5,wherein the material has a layered perovskite structure.
 7. Materialaccording to claim 1, wherein it has an average particle size of 0.5 μmto 1 μm, for example 0.8 μm to 0.9 μm, or 0.5 μm to 0.6 μm.
 8. Materialaccording to claim 1, further comprising a compound of formula (3)LaNi0.6Fe0.4O3−δ (3) where 0<δ≤0.05.
 9. Method for producing a materialfor an electrode, according to claim 1, comprising the steps of (a)mixing the oxides of the elements Pr, Nd, Ni, Co, La according to thedesired compound of formula (1) or (2), (b) drying the mixture from step(a), (c) annealing the mixture at a temperature of 1000° C. to 1400° C.for 4 hours to 20 hours in air.
 10. Method according to claim 9, whereinthe mixing of the oxides in step (a) is carried out using a ball mill inthe presence of a liquid for 2 to 6 hours.
 11. Method according to claim9, wherein the drying in step (b) is carried out at 18° C. to 100° C.,for 8 hours to 24 hours.
 12. Method according to claim 9, wherein theannealing in step (c) is carried out at a temperature of approximately1300° C. for approximately 12 hours.
 13. Method according to claim 9,wherein, after step (c), the average particle size is adjusted to 0.5 μmto 1 μm.
 14. Use of the material according to claim 1 as an electrodematerial.
 15. Use according to claim 14, wherein the electrode is an airelectrode or oxygen electrode for a fuel cell or electrolysis, inparticular for high temperature electrolysis.
 16. Method according toclaim 10, wherein the mixing of the oxides in step (a) is carried outusing a ball mill in the presence of a liquid for 3 to 5 hours. 17.Method according to claim 11, wherein the mixing of the oxides in step(a) is carried out using a ball mill in the presence of a liquid for 4hours.
 18. Method according to claim 9, wherein the drying in step (b)is carried out at 18° C. to 100° C. for 12 hours.
 19. Material accordingto claim 2, wherein δ has the values 0.25, 0.28 or 0.3.
 20. Material foran electrode, the material consisting of a compound of formula (1)M2Ni1−xCoxO4+δ  (1)and/or of formula (2)La1−yMyNi1−xCoxO4+δ  (2) where M represents Pr and/or Nd, 0.0≤x≤0.2,0.25≤δ≤0.3 and 0<y≤0.5.